1. Field of the Invention
The present invention relates to high power pulsed microwave amplifiers and, in particular, to a correction system utilizing a digitally programmable phase shifter and attenuator and associated control circuitry to minimize unwanted phase and amplitude variations in the output pulses generated by the pulsed microwave amplifier.
2. Description of the Prior Art
Devices for producing repeatable pulses of microwave energy at very high peak power levels typically include a pulsed microwave amplifier which amplifies a signal produced by a low power microwave generator. The energy required for signal amplification may be derived from a high energy electron beam that is directed into the amplifier. Klystrons, relativistic klystrons, free electron lasers and cyclotron auto-resonant masers are examples of pulsed microwave amplifiers of this kind.
Amplitude and phase stability at the output of pulsed microwave amplifiers of this kind is strongly dependent on the stability of the driving electron beam. Variations in electron beam energy can cause the phase of the microwave output energy to vary during the course of a pulse. Variations in beam current may produce amplitude fluctuations in the high power microwave output during an output pulse.
The variations are tolerable in some applications that require a very high power source of microwave energy such as in nuclear fusion plasma heating installations. Greater stability is needed in other usages of such apparatus as in certain types of charged particle accelerators.
Recently developed pulsed microwave amplifiers typically operate with pulse durations as short as a few tens of nanoseconds. Conventional feedback correction techniques cannot be used to stabilize the phase and/or amplitude of the output of high power pulsed microwave sources during each pulse in instances where the duration of each pulse is less than about 100 nanoseconds. These devices frequently have physical sizes and resulting long signal paths that make it impractical, if not impossible, to correct phase and amplitude variations within a pulse using conventional feedback techniques and systems.
A prior art system that seeks to provide a solution to the above problem provides a method for stabilizing the output of an electron beam driven pulsed microwave amplifier of the type having an input for receiving a low power microwave signal that is to be amplified and an output for delivering periodic output pulses of amplified microwave energy to a load. The phase and amplitude characteristics of the microwave energy that is to be stabilized is sensed at several time points of the output pulses of the high power amplifier to produce a sequence of output signals. The output signals are compared with predetermined reference signals to produce a sequence of error signals that are indicative of variations of the characteristics at the amplifier output during individual ones of the output pulses. The error signals obtained during each of the sequence of output pulses are used to produce and store correction signals. The correction signals which were stored during prior output pulses are used to modify the low power microwave input signal during subsequent output pulses to suppress the variations during the subsequent output pulses. This method, in essence, utilizes a feed-forward correction technique.
The apparatus includes means for comparing the output signals with the reference signals to produce a sequence of error signals that are indicative of variations of both the amplitude and phase characteristics at the amplifier output during individual ones of the output pulses. Additional means store correction signals that include the error signals during each amplifier output pulse and use the correction signals that were stored during prior output pulses to modify the characteristics in the low power microwave input signal during subsequent output pulses to suppress the variations in the subsequent output pulses. In this prior art system, a voltage controlled phase shifter is coupled into the low power microwave signal path. A phase difference detector has a first input coupled to the reference signal and a second input coupled to the amplifier output. The output of the phase difference detector exhibit a voltage difference that is a function of the magnitude of phase error at the amplifier output. A phase correction signal storage means is coupled to the phase difference detector and has means for storing a plurality of momentary values during the course of each pulsed amplifier output pulse. The momentary values are indicative of the value of the voltage difference at successive different times in the course of the output pulse combined with the momentary value that was stored at the corresponding time during a preceding output pulse. Further components include means for sequentially reading out the previously stored momentary values during each output pulse in the form of a series of voltage levels which jointly provide the control voltage for the voltage controlled variable phase shifter. The system also provides apparatus for suppressing amplitude variations at the output of the pulsed microwave amplifier during individual output pulses. A voltage controlled amplitude modulator, such as an attenuator or other device displaying a voltage controlled amplitude characteristic, is coupled into the input signal path. An amplitude difference detector has a first input coupled to the reference signal and a second input coupled to the output of the amplifier and has an output which exhibits a voltage difference that is a function of the magnitude of amplitude error at the amplifier output relative to the input signal. An amplitude correction signal storage means is coupled to the amplitude difference detector and has means for storing a plurality of momentary values during the course of each amplifier output pulse. The momentary values are indicative of the value of the voltage difference at successive different times in the course of the output pulse combined with the momentary value that was stored at the corresponding time during a preceding pulse. Means are provided to sequentially read out the previously stored momentary values during each output pulse in the form of a series of successive voltage levels which jointly provide the control voltage for a voltage controlled variable signal amplitude modulator.
Although this prior art feed-forward correction system purports to provide advantages over conventional feedback correction systems, it has a number of drawbacks. In particular, the system uses analog type components, such as the voltage controlled phase shifter and amplitude modulator described hereinabove which substantially reduces the response time of the correction system. Analog output switches or multiplexers are also very slow. These analog circuit characteristics reduce the effectiveness of correcting the variations in the narrow pulse, high frequency amplifier output.
What is thus desired is to provide a method and apparatus to correct unwanted variations in the phase and amplitude of a radio frequency (RF) pulse generated by a beam driven high power RF source in a manner which is faster and more accurate and reliable than previously proposed correction systems.